Electron beams are used in a variety of applications including, but not limited to irradiation of packaging materials for sterilization purposes. For example, packaging materials such as cartons used to hold liquids for human consumption are sterilized using electron beam irradiation. To provide on-line control of the intensity of the electron beam, and to monitor uniformity variations, electron sensors are used for dose irradiation measurement. A signal from the sensor is analyzed and fed back into an electron beam control system as a feedback control signal. In the sterilization of packaging material, such sensor feedback can be used to assure a sufficient level of sterilization. Different levels of sterilization can be chosen depending on how long shelf-life is desired and whether the distribution and storage of the packages is made in chilled or ambient temperature.
One kind of existing sensor for measuring electron beam intensity, based on direct measuring methods, uses a conductor placed within a vacuum chamber. The vacuum chamber provides isolation from the surrounding environment. Because the vacuum-based sensors are relatively large, they are located at positions outside the direct electron beam path to avoid shadowing of target objects. Shadowing can, for example, preclude proper irradiation (and thus, proper sterilization) of packaging material. Therefore, these sensors rely on secondary information from a periphery of the beam, or information from secondary irradiation, to provide a measurement.
In operation, electrons from the electron beam which have sufficient energy will penetrate a window, such as a titanium (Ti) window of the vacuum chamber and be absorbed by the conductor. The absorbed electrons establish a current in the conductor. The magnitude of this current is a measure of the number of electrons penetrating the window of the vacuum chamber. This current provides an indirect measure of the intensity of the electron beam at the sensor position.
A known electron beam sensor having a vacuum chamber with a protective coating, and an electrode representing a signal wire inside the chamber, is described in published U.S. patent application No. US 2004/0119024. The chamber walls are used to maintain a vacuum volume around the conductor. The sensor described includes a vacuum chamber window accurately aligned with the electrode to sense the electron beam density. The sensor is configured for placement at a location, relative to a moving article being irradiated, opposite the electron beam generator for sensing secondary irradiation. A similar electron beam sensor is described in patent publication WO 2004 061890. In one embodiment of this sensor the vacuum chamber is removed and the electrode is provided with an insulating film. The insulating film is provided to avoid influence from electrostatic fields and plasma electrons created by the electron beam from substantially influencing the electrode output.
U.S. Pat. No. 6,657,212 describes an electron beam irradiation processing device wherein an insulating film is provided on a conductor, such as a stainless steel conductor, of a current detection unit placed outside a window of an electron beam tube. A current measuring unit includes a current meter that measures current detected. This patent describes advantages of a ceramic coated detector.